The invention relates to a colour display tube comprising an evacuated envelope which is composed of a neck, a cone and a display window. In the neck a system of electron guns with which three electron beams situated with their axes in one plane are generated. The beams are focused, by means of a focusing lens field, on a display screen which is provided on the inside of the display window. The focusing lens field is formed between two electrodes of the system of electron guns situated one behind the other along the axes. The electrodes comprise outer rims facing each other and within these rims three apertures through which the electron beams pass are provided symmetrically with respect to the plane in a recessed part extending substantially at right angles to the axes.
Such a colour display tube is known from U.S. Pat. No. 4,370,592. It is stated in the Patent Specification that the focusing lens field generated in this manner is still astigmatic and the electron beams are focused vertically more strongly than horizontally. As described in the Patent Specification, the astigmatism is corrected by providing the second electrode of the focusing lens with a plate having an elongate slot which is provided on the side of the display screen. The slot is situated symmetrically with respect to the plane through the beam axes. However, it has been found that the corrections of the horizontal/vertical astigmatism are not sufficient since the quality of the spot on the display screen leaves something to be desired.
United Kingdom Patent Application No. 2,112,564 also describes such a colour display tube. The horizontal/vertical astigmatism is eliminated by making the apertures in the recessed part elongate. This is also not sufficient to get smaller spot dimensions.